A wireless multihop communications network, such as a mesh network, includes a set of node devices capable of exchanging messages with one another over a wireless medium, typically using radio frequency (RF) communications. Each of the node devices can be primarily a communications device or, alternatively, the communications functionality may be secondary to its primary function. For example, in a given node, the communications circuitry can be a part of a device such as a computer system, a smart appliance, a vehicle, a media device, a piece of industrial equipment (e.g., an instrument, machine, sensor, actuator), and the like.
In a multi-hop mesh architecture the node devices are uniquely addressable, and are capable of selecting among sets of alternative intermediate node devices in establishing a communications path to route messages from an originating node device toward the intended destination node. The particular path from the origination device to the destination is a function of the criteria defined within the routing protocol applied within the network. In general, each of the node devices can be an originating and destination node device as well as act as a relay device. Thus, node devices perform both, message forwarding, and message origination/consumption functions. This also means that communication link activity can be variable across the network and quite heavy at certain devices of the network as a function of the source-destination traffic patterns and the available intermediate devices' connectivity.
Wireless multi-hop mesh networks in particular face other challenges. For instance, wireless links may not always be reliable: there may be intermittent interfering signals, intermittent obstructions, including large movable objects (e.g., vehicles) moving in and out of the transmission path, weather affecting the quality of radio signal propagation, etc., affecting the signal strength of transmissions seen by the receiving node. Also, certain node devices may be situated near the limits of their radio's communication range, relative to a node neighbor, which further compounds signal reception challenges.
In ad hoc, multi-hop RF mesh networks nodes form dynamic associations with their local neighbors. In general, network connectivity is established based on node information broadcasts and dedicated routing information communications exchanges between neighbor devices. Evaluated link connectivity and routing information exchanges allow each node to derive optimal forwarding paths to other nodes in the network and to gateway nodes that act as the access points into and out of the RF network.
In a multi-hop network, nodes can receive or initiate neighbor connection requests for network route maintenance or to relay traffic across the network at any time. However, with a single RF transceiver, each node is limited to communicating with a single neighbor at any given time. Where nodes operate in a single communications mode (as defined, for example, by particular frequency channelization, transmission modulation, or multi-access methods), an idle node is able to continuously monitor for neighbor connection requests and can initiate its own connection to a selected neighbor at any time.
Where nodes are capable of operating in more than one transmission mode, such as where nodes can selectively transmit using different modulation techniques, there needs to be a higher degree of coordination or organization if nodes are to be able to asynchronously connect with neighbors with minimum delay when a connection needs to be established for routing exchange, message relay, or other communications exchange.
One known way to coordinate the multiple modes in a multi-hop mesh network relies on some form of centralized timing control in which network-wide time synchronization prescribes dedicated time periods during which the different transmission modes can occur at particular nodes. For ad hoc networks, where there can be dynamic, overlapping sets of associations between groups of nodes that are within radio range of one another, such a centralized coordination approach would be unfeasibly restrictive. It would be inefficient and impractical to pre-define periods for communications in one mode or the other for each and every node across the network. If such an approach were attempted for a generalized communications network, the asynchronous, stochastic nature of initiated network connections and data flows would result in significant network delays in the transfer of data between source-destination points. At each hop traffic would be forced to wait for the time availability of the particular mode in communicating from one neighbor to the next along the forwarding path. Furthermore, where nodes in a multi-hop or mesh network environment can have tens or even hundreds of neighbors depending on network size, any form of centralized control would severely constrain the practical limits of network scalability.
A solution is needed for a practical, adaptive network, in which multi-mode communications can be utilized effectively, particularly under dynamically changing circumstances and traffic exchanges that may be associated with a general ad hoc network.